1. Field of the Invention
The present invention is related to media pressure vessels for bulk abrasive blaster systems and is specifically directed to the closure assembly for such vessels.
2. Description of the Prior Art
The function of a bulk abrasive blaster system is to provide a controlled mixture of dry abrasive or non-abrasive media and compressed air to a blast nozzle. The abrasive blast stream may be used to prepare the surface being blasted for the application of a protective coating or may be used for simple removal of unwanted surface defects, or other typical blasting operations. The bulk abrasive blaster is one of the components of the abrasive blasting system. The system typically comprises an air compressor, a moisture removal device, the bulk abrasive blaster, a blast line and a blast nozzle.
In the most typical applications, the bulk abrasive blaster comprises a pressure tank or pressure vessel with a top closure assembly adapted to be placed in communication with the outlet of an abrasive storage hopper for filling the tank when the abrasive or media therein is depleted. A pressure system is utilized for pressuring the tank when it contains an operable amount of abrasive. Once the tank is pressurized, the abrasive or media flows through an outlet and into a blast line where it is propelled through the blast nozzle.
When the system is shut down, or in blowdown condition, the pressure valve is closed and a blowdown valve is opened to relieve the pressure in the vessel and permit it to achieve ambient pressure levels.
It is important in the operation of the system that the tank closure assembly not be opened at any time when the pressure tank is at a pressure higher than the ambient external pressure. This is particularly important when the system is operated in such a manner the bulk abrasive blaster system, including the pressure tank, is maintained in a pressurized state when in an off condition. However, caution must be taken at all times to make certain the vessel is at ambient external pressure before the closure assembly is opened for any reason.
In normal depressurizing operation, the air inlet valve on the tank is closed. The blowdown valve on the tank is then opened to relieve the pressure in the tank. Once airflow from the blowdown valve stops and the tank is depressurized, the closure assembly may be opened. Typically a pressure indication valve is provided near the closure assembly to provide a check point for confirming that the vessel is at ambient pressure before opening the closure assembly.
Prior art closure assemblies are designed to minimize the ability to open the tank when it is pressurized. These typically include a closure hub installed on an opening in the top of the tank and a closure head for opening and closing the opening in the hub and the tank.
The hub is generally cylindrical and is welded on the vessel. A hinged closure head is designed to fit on the closure hub for opening and closing the vessel. One or more swing bolts are mounted on the hub and designed to swing into and out of engagement with a holding lug on the closure head. Locking nuts are placed on the outer end of the bolts and are tightened down against the holding lugs for locking the closure head against the closure hub. Typically, the bolts are mounted in a cam lock assembly wherein the swing bolts and nuts, once assembled, may be tightened against the holding lug by swinging a camlock handle from an opened position to a closed, locked position, for generating tension in the swing bolts.
When the tank is pressurized, the closure head will attempt to move up and away from the closure hub, increasing the tension on the bolts. When the tank is depressurized, the closure head will move into its fully seated position, reducing the tension on the bolts. In proper application, the camlock handle may not be moved from the locked to the open position when the tank is pressurized, but is easy to swing from the locked to the open position when the tank is depressurized.
This type of closure assembly serves as an access port that can be opened without the use of tools. The safe use of the closure requires that the pressure vessel first be completely free of internal air pressure prior to opening the closure. A hazardous situation occurs when the operator fails to vent the air pressure prior to attempting to open the closure. The compressed air inside the tank can contain a dangerously high level of energy which can propel objects. This air pressure will also cause the failure of the closure assembly if some of the swing bolts are moved into the open position with the presence of air pressure within the pressure tank or pressure vessel.
The camlock assembly is a first level safety feature that alerts the operator of the presence of air pressure within the tank by allowing air to leak from the closure assembly provided that the closure is opened in the proper sequence. The proper sequence requires that all camlock handles be swung into the open position prior to moving any of the swing bolts, thereby reducing the tension on the swing bolts. This allows the closure head to slightly rise and leak air pressure, thereby alerting the operator to the presence of air pressure.
During the incorrect opening sequence the operator can swing one camlock handle and then proceed in moving the swing bolt out of the holding lug. And then proceed to open the next camlock assembly in this incorrect method. If there is air pressure present the closure head will not raise to alert the operator since the remaining swing bolt will hold the closure head down. If the operator proceeds in incorrectly opening the camlock assemblies the stress induced on the remaining swing bolts by the air pressure on the closure head can cause the failure of these swing bolts.
Depending on the conditions and sequence, the swing bolt may not easily move from the holding nut due to the air pressure within the tank. The operator may not recognize this as an indication of the presence of air pressure and then proceed to improperly use a hammer or other such device to aid in knocking out the swing bolt from the holding nut which will result in failure in the event that air pressure is in fact within the pressure tank.
While this system is very effective when used properly, it is desirable to provide a secondary safety interlock that will require proper sequence of opening.